1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices and particularly thyristors.
This application is considered an improvement on the method described and claimed in U.S. Pat. No. 4,075,037, granted Feb. 21, 1978 and assigned to the same assignee as the present application.
It has recently been demonstrated to irradiate semiconductor devices to modify the electrical characteristics in various ways. See, e.g., U.S. Pat. Nos. 3,809,582; 3,840,887; 3,852,612; 3,872,493; 3,877,977; 3,881,963; 3,881,964; 3,990,091; 4,040,170; 4,056,408; 4,075,037; 4,076,555 and U.S. Pat. application Ser. Nos. 929,624 filed July 31, 1978 now U.S. Pat. No. 4,224,083; 972,302 filed Dec. 22, 1978 now U.S. Pat. No. 4,240,844 and 000,936 (filed Jan. 4, 1979), all of which are assigned to the same assignee as the present application.
Certain semiconductor devices must be switched from a high current forward conducting mode to a high voltage reverse blocking mode during their operation. The most common of these is thyristors. More complex semiconductor devices containing a thyristor component include diacs, triacs, reverse switching rectifiers and reverse conducting thyristors.
When a thyristor and particularly a power thyristor is in the forward conducting state excess carrier concentrations and corresponding excess charge are present in the base regions, and particularly the anode base region which generally has a lower impurity concentration than the cathode base region. During reverse recovery of the thyristor, the excess charge called the reverse recovery charge (Qrr) must be removed by carrier recombination and diffusion, limiting the reverse blocking characteristics of the thyristor. The amount of the reverse recovery charge is a function of the devices' parameters, most notably, the minority carrier lifetime in the base regions and the current gain across the anode base region.
For many thyristor applications it is desirable to have the reverse recovery charge (Qrr) as low as possible or tailored to a particular value. In U.S. Pat. No. 4,075,037, it is proposed to reduce the excess charge by reducing the minority carrier lifetime in the base regions and particularly in the cathode base region by irradiating the device preferably with electron radiation. Although this method has been successful, the dosage must be carefully controlled with this technique because the reduction in minority carrier lifetime and reduction in excess charge is accompanied by an increase in forward voltage drop (V.sub.TM) and leakage current. A trade off must be effected particularly between reduction in excess charge and an increase in forward voltage drop.
The present invention overcomes these difficulties and provides a method of reducing the reverse recovery charge (Qrr) of a thyristor with minimal changes in forward voltage drop (V.sub.TM) and leakage current.
In U.S. Pat. No. 4,056,408, a method is described and claimed for reducing the switching time of semiconductor devices, including thyristors, by irradiating with nuclear radiation and preferably protons or alpha particles. The energy level from the radiation source is adjusted to provide defect generation adjacent a blocking PN junction. It teaches, contrary to the present invention, that the maximum defect generation should be adjacent the blocking PN junction, between the cathode base and anode base regions.